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<br />PREHISTORIC FLOODS IN POUDRE CANYON <br /> <br /> z.'-/?9 <br />I " , " ( <br />,- <br /> /i.. <br /> ( \':." <br /> CKt'n <br />~- - o 3 1992 <br /> <br />Progress Report, February 1992 <br /> <br />Ellen E. Wohl <br />Department of Earth Resources <br />Colorado State University <br /> <br />I. PROJECT SUMMARY <br /> <br />Fine-grained flood sediments deposited along the lower <br />reaches of poudre Canvon have been used to reconstruct flood <br />magnitudes and freque~cies in the canyon. Four depositional <br />sites located near Upper picnic Rock were used to define a 2-km- <br />long study reach. Elevations of flood deposits provided a <br />minimum estimate of peak stage. Twenty-three surveyed cross <br />sections were used to characterize channel geometry in this <br />reach, and flood discharges were routed through the reach using <br />the US Army Corps of Engineers' step-backwater model, HEC-2. <br />Simulated water-surface profiles were then compared to water- <br />surface profiles defined from field evidence, to produce a best <br />estimate of flood discharge. Flood sediments indicated multiple <br />peak discharges during the last 6500 years, ranging from 530,000 <br />to 3500 ft3s-1. Three radiocarbon ages from charcoal contained <br />within the flood sediments indicate an age range of 700 to 6500 <br />years before present, although the complex stratigraphy of the <br />deposits requires further geochronologic control before accurate <br />interpretations can be made of the number of floods recorded in <br />the sediments. <br /> <br />Proposed future work includes an evaluation of two potential <br />causes of the floods recorded in lower Poudre Canyon; breaching <br />of a debris-flow dam, and intense rainfall. When the flood <br />chronology for lower Poudre Canyon is expanded, we will also <br />apply this information to flood-frequency analysis. <br />